The present invention relates to an optical gain equalizer for flattening gain wavelength dependency of an optical amplifier used for optical transmissions, etc., a method for producing the optical gain equalizer, an optical amplifier utilizing the optical gain equalizer, and a wavelength multiplexed light transmission system utilizing the optical amplifier.
A wavelength multiplexing system (optical wavelength division multiplexed transmission system) is available as a system to achieve high bit rate transmissions in optical transmission systems. The system is such that optical signals of a plurality of wavelengths different from each other are multiplexed and transmitted in a single optical transmission line consisting of, for example, optical fibers. Various research has been carried out on a wavelength multiplexed light transmission system utilizing the system.
Optical semiconductor amplifiers in which optical semiconductors are utilized have been conventionally researched. Recently, an optical fiber type optical amplifier has been researched, in which a rare metal doped optical fiber such as an erbium doped optical fiber, etc., is used as an amplifying medium, and such an optical fiber type optical amplifier has rapidly been achieved for practical applications. Such an optical amplifier can collectively amplify light of wavelengths in a gain wavelength band. Therefore, it is highly expected that, by applying such an optical amplifier to the wavelength multiplexed light transmission system, a high bit rate and long haul transmission system is achieved.
However, in both an optical fiber type optical amplifier and an optical semiconductor amplifier, the gain thereof has a wavelength dependency. If such a wavelength multiplexed light as described above is collectively made incident into an optical amplifier since the gain size differs, depending on wavelengths of light incident into an optical amplifier, the intensity of light outputted from the optical amplifier will differ, depending on the wavelengths. Due to differences in the outputted light intensity resulting from the wavelengths, a problem of crosstalk between the respective wavelengths occurs. Also, if the intensity of light outputted from an optical amplifier differs, another problem occurs in setting the receiving level, whereby the reception level of a wavelength multiplexed light receiving portion which receives the outputted light must be established at different values depending on, for example, the wavelength of the received light.
Therefore, in order to compensate a gain wavelength dependency of such an optical amplifier, such a method has been proposed, in which a transmission light filter type optical gain equalizer to flatten the gain wavelength dependency of the optical fiber type optical amplifier is produced by a combination of Fabry-Perot etalon filters, and the optical gain equalizer is inserted into an optical fiber type optical amplifier. The method is described in, for example, Japanese Laid-open Patent Publication No. 289349 of 1997. The proposed method flattens gains of an optical amplifier by expanding a gain curve of the optical amplifier with respect to signal light wavelengths to a Fourier series and combining an etalon filter having since-wave type loss characteristics of the same amplitude and cycminimumhose of the since-wave type loss characteristics obtained by the expansion.
However, the following problems exist in the method proposed above. That is, the equalizing characteristic T if etalon is expressed by expressions (1), (2) and (3), where the reflection index is R, the incident angle is xcex8 (where the incident angle perpendicularly incident with respect to the filter surface is 0), the refractive index of a filter substrate is n, the thickness of the filter substrate is d, the light speed is c, and the wavelength of the incident light is frequency f. The equalizing characteristic T of etalon is different in a waveform from respective cosine-wave components expanded in the Fourier series. Thus, since, in the abovementioned method, an optical gain equalizer is in an attempt to be formed by an expansion which is not inherently mathematically guaranteed, there remain components which are not compensated even though the gain wavelength dependency of an optical amplifier is attempted to be compensated.                     T        =                              -            10                    xc3x97                                    log              10                        ⁡                          [                              1                +                                                      m                    i                                    ⁢                                                            sin                      2                                        ⁡                                          (                                                                                                    2                            ⁢                            π                            ⁢                                                          xe2x80x83                                                        ⁢                            f                                                    2                                                ⁢                                                  xe2x80x83                                                ⁢                                                  m                          2                                                                    )                                                                                  ]                                                          (        1        )                                          m          1                =                              4            ⁢            R                                              (                              1                -                R                            )                        2                                              (        2        )                                          m          2                =                              2            ⁢            nd            ⁢                                          1                -                                                      sin                    2                                    ⁢                                      θ                    /                                          n                      2                                                                                                    c                                    (        3        )            
A waveform of any optional form can be expressed in the form of the cosine- or sine-wave infinite series. Therefore, it is considered that, if a very large number of etalon filters are used although the gain wavelength dependency of an optical amplifier cannot be completely compensated, characteristics close to the since-wave loss characteristics obtained by expanding the abovementioned Fourier series can be obtained. Actually, however, it is impossible to form an optical gain equalizer by a very large number of etalon filters which are nearly infinite. For convenience in production, the number of etalon filters is four, at most, which is the limit in view of production. Accordingly, in actuality, the since-wave loss characteristics cannot be obtained by the method proposed above, wherein it is also impossible to only effectively compensate the gain wavelength dependency of an optical gain amplifier.
That is, since, in the abovementioned prior art method, infinite term components obtained by expanding the gain curve of an optical amplifier with respect to signal light wavelengths are in an attempt to be achieved by definite terms (that is, an attempt to be achieved by etalon filters, the usage number of which is limited), the gain wavelength dependency cannot be effectively compensated.
Further, in the case of Fourier series expansion, parameters of expanding terms differ in wavelength cycles selected in an attempt to compensate by an optical gain equalizer, that is, basic cycles of the Fourier series. Accordingly, a remarkably great amount of labor is required in order to determine design matters of etalon, which are obtained by using the Fourier series expansion.
The present invention was developed to solve the abovementioned problems and shortcomings in the prior art methods. It is therefore a first object of the invention to provide an optical gain equalizer which is capable of effectively compensating a gain wavelength dependency of an optical amplifier by a simple method, and a method for producing the same optical gain equalizer. It is a second object of the invention to provide an optical amplifier device having almost no gain wavelength dependency by proposing such an optical gain equalizer, and further it is a third object of the invention to provide a wavelength multiplexed light transmission system which is capable of suppressing the wavelength dependency of light intensity at the receiving side, by using an optical amplifier device in which the optical gain equalizer is used.
In order to achieve the above objects, the invention has the characteristic structures described below. That is, a first aspect of a method for producing an optical gain equalizer according to the invention is featured in that, where it is assumed that a loss wavelength characteristic which completely compensates the gain wavelength dependency of an optical amplifier device in the predetermined set range of wavelengths including at minimum a usage range of wavelengths is the ideal loss wavelength characteristic, N (N: a positive integer) wavelengths different from each other, which are optionally selected in the usage range of wavelengths is xcexi (i is an integer which increases sequentially from 1 like 1, 2, 3, . . . N), and design values of parameters which determine the loss wavelength characteristic of an optical component are a1, . . . , am having a nonlinear coupling, the respective parameters a1, . . . , am of the optical component are determined by using a nonlinear fitting method so that the total sum of a square error of a loss value yi at the respective wavelengths xcexi of the ideal loss wavelength characteristic and a loss value xcexi at the respective wavelengths in which the design value of the parameter of the optical component is made into a parameter, and an optical gain equalizer is produced by using an optical component having the loss wavelength characteristic which is determined by using the determined parameters a1, . . . , am.
A second aspect of a method for producing an optical gain equalizer according to the invention is featured in that, in addition to the first aspect of a method for producing the optical gain equalizer, a reference gain value which is smaller than the minimum value of gain in the range of usage wavelength of an optical amplifier is predetermined, and a loss wavelength characteristic which counterbalances gain greater than the reference gain value is made into an ideal loss wavelength characteristic.
The first aspect of a method for producing an optical gain equalizer according to the invention is featured in that, where it is assumed that a loss wavelength characteristic which completely compensates the gain wavelength dependency of an optical amplifier device in the predetermined set range of wavelengths including at minimum a usage range of wavelengths is the ideal loss wavelength characteristic, N (N: a positive integer) wavelengths different from each other, which are optionally selected in the usage range of wavelengths is xcexi (i is an integer which increases sequentially from 1 like 1, 2, 3, . . . N), and design values of parameters which determine the loss wavelength characteristic of an optical component are a1, . . . , am having a nonlinear coupling, the respective parameters a1, . . . am of the optical component are determined by using a nonlinear fitting method so that the total sum of a square error of a loss value yi at the respective wavelengths xcexi of the ideal loss wavelength characteristic and a loss value xcexi at the respective wavelengths in which the design value of the parameter of the optical component is made into a parameter, and an optical component having a loss wavelength characteristic which is determined by the respective parameters is used as a compensating component of a gain wavelength dependency.
Further, an optical component in the optical gain equalizer is featured in that it is composed of an etalon filter element or Mach-Zehnder interference type element.
A first aspect of an optical amplifier device according to the invention is featured in that it has an optical amplifier for amplifying wavelength multiplexed light, and an optical gain equalizer according to the invention is connected to either one of the input side or output side of the optical amplifier.
Also, in another aspect, the optical amplifier device according to the invention is featured in that a plurality of optical amplifiers are disposed in tandem.
Further, in still another aspect, an optical amplifier of the optical amplifier device according to the invention is featured in being an optical semiconductor amplifier or an optical filter type amplifier.
Further, a wavelength multiplexed light transmission system according to the invention is featured in that the system comprises a wavelength multiplexed light transmission portion which multiplexes and transmits light of a plurality of wavelengths different from each other, a wavelength multiplexed light transmission line which transmits wavelength-multiplexed light transmitted from the wavelength multiplexed light transmission portion, and a wavelength multiplexed light receiving portion which receives wavelength-multiplexed light transmitted through the wavelength multiplexed light transmission line, wherein one or more optical gain equalizers according to the invention is provided in the wavelength multiplexed light transmission line.
The method for producing an optical gain equalizer according to the invention is such that, where it is assumed that design values of parameters which determine a loss wavelength characteristic of optical components of an optical gain equalizer to compensate gain wavelength dependency of an optical amplifier are made into a1, . . . , am which have a nonlinear coupling, and a loss value at respective N wavelengths xcex1 different from each other, which are optionally selected in a range of usage wavelengths, of a loss wavelength characteristic which completely compensates the gain wavelength dependency of an optical amplifier device in the predetermined set range of wavelengths including at minimum a usage range of wavelengths of the ideal loss wavelength characteristic is yi, and the respective parameters a1, . . . , am of the optical component are determined by a nonlinear fitting method so that the total sum of a square error of a loss value yi at the respective wavelengths xcexi and a loss value at the respective wavelengths xcexi in which the design value of the parameter of the optical component is made into a parameter becomes the minimum value, wherein an optical gain equalizer is produced by using an optical component having the loss wavelength characteristic which is determined by using the determined parameters a1, . . . , am. Therefore, the loss wavelength characteristic of an optical component can be made into the loss wavelength characteristic closest to the ideal loss wavelength characteristic.
In other words, by determining the parameters a1, . . . , am of an optical component as described above, it becomes possible to achieve an ideal loss characteristic at the highest possible accuracy. This can be mathematically made clear, wherein, by applying the mathematical fact to a method for producing an optical gain equalizer, it is possible to produce an optical gain equalizer in which optical components having the loss wavelength characteristic closest to the ideal loss wavelength characteristic are used. Accordingly, an optical gain equalizer which has been produced by the method is capable of very efficiently compensating the gain wavelength dependency of an optical amplifier.
Furthermore, the method for determining the respective parameters of the respective optical components does not require such a large amount of labor as in the case where prior art Fourier series expansion is used. Therefore, an optical gain equalizer according to the invention is capable of very efficiently compensating the gain wavelength characteristic of an optical amplifier by only a simple method. Also, by forming an optical amplifier device using the optical gain equalizer, an excellent optical amplifier device which has almost no gain wavelength dependency in a range of usage wavelengths can be formed. Also, if a wavelength multiplexing transmission system is constructed by using an optical amplifier device in which the optical gain equalizer is used, it becomes possible to suppress and prevent the wavelength dependency of the received light intensity.
In particular, according to a method for producing an optical gain equalizer of the invention, in which a reference gain value which is smaller than the minimum value of gain in a range of usage wavelengths of an optical amplifier is predetermined, and a loss wavelength characteristic which counterbalances any gain greater than the reference gain value is made into the ideal loss wavelength characteristic, it is possible to easily obtain the ideal loss wavelength characteristic securely.
And, since an optical gain equalizer of the invention is formed by using optical components determined by respective parameters a1, . . . , am obtained by the method as described above, it is possible to very easily produce the optical gain equalizer. Furthermore, it is possible to obtain an excellent optical gain equalizer which can very efficiently compensate the gain wavelength dependency of an optical amplifier.
Also, by utilizing optical components, which form an optical gain equalizer, consisting of an etalon filter element or Mach Zehnder interference element, an optical gain equalizer of the invention which can bring about such excellent effects and advantages as described above can be easily obtained securely.
Further, since an optical amplifier device of the invention is formed by using an excellent optical gain equalizer as described above, it can be made into an excellent optical amplifier device which has almost no gain wavelength dependency in a range of usage wavelengths.
Still further, according to an optical amplifier device of the invention, which is provided with a plurality of optical amplifiers, it is possible to further efficiently amplify wavelength-multiplexed light by a plurality of optical amplifiers.
And, according to an optical amplifier device of the invention, in which at minimum one of the optical amplifiers is made into an optical semiconductor amplifier, optical amplifiers can be very easily formed by utilizing a prior art semiconductor technology, whereby an optical amplifier device can be easily formed. Also, according to an optical amplifier device of the invention, in which at minimum one of the optical amplifiers is made into an optical fiber type amplifier, light can be directly amplified without being converted to electric signals. Therefore, it is easy to handle, and the optical amplifier device is advantageous in actual applications.
In addition, according to a wavelength multiplexing transmission system using the optical amplifier device provided with an optical gain equalizer according to the invention, it is possible to effectively suppress the wavelength dependency of the received light intensity. Therefore, according to a wavelength multiplexing transmission system of the invention, the problem of crosstalk of the received light can be solved, and such an inconvenience wherein the receiving level of light of the respective wavelengths that must be adjusted at the wavelength multiplexed light receiving portion can be removed. Therefore, the configuration of the system can be facilitated, and it is possible to obtain an excellent system-which enables high bit rate and long haul transmissions.